Imaging catoptric EUV projection optical unit

ABSTRACT

An imaging catoptric optical unit has at least four mirror, which image an object field in an object plane into an image field in an image plane. A first chief ray plane of the optical unit is prescribed by propagation of a chief ray of a central object field point during the reflection at one of the mirrors. A second chief ray plane of the optical unit is prescribed by propagation of the chief ray of the central object field point during the reflection at one of the other mirrors. The two chief ray planes include an angle that differs from 0. In an alternative or additional aspect, the imaging optical unit, considered via the image field, has a maximum diattenuation of 10% or a diattenuation that prefers a tangential polarization of the imaging light for a respectively considered illumination angle. The result of both aspects is an imaging optical unit in which bothersome polarization influences are reduced during the reflection of imaging light at the mirrors of the imaging optical unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of, and claims benefit under35 USC 120 to, international application PCT/EP2012/069158, filed Sep.28, 2012, which claims benefit under 35 USC 119 of German ApplicationNo. 10 2011 083 888.0, filed Sep. 30, 2011. International applicationPCT/EP2012/054664 also claims priority under 35 USC 119(e) to U.S.Provisional Application No. 61/541,127, filed Sep. 30, 2011. Thecontents of international application PCT/EP2012/069158 and Germanpatent application 10 2011 083 888.0 are incorporated by reference.

The invention relates to an imaging catoptric EUV projection opticalunit and an imaging catoptric optical unit.

Such imaging optical units are known from US 2010/0231886 A1. Suchimaging optical units are part of a projection exposure apparatus andare used when the structure of a reticle is imaged in projectionlithography for producing integrated circuits.

It is an object of the present invention to develop an imaging opticalunit of the type specified at the outset such that bothersomepolarization influences are reduced.

According to a first aspect, this object is achieved according to theinvention by an imaging catoptric EUV projection optical unit with atleast four mirrors which image an object field in an object plane intoan image field in an image plane. The imaging optical unit has a firstchief ray plane, which is defined by propagation of a chief ray of acentral object field point during the reflection at a mirror. Theimaging optical unit includes a second chief ray plane, which is definedby propagation of the chief ray of the central object field point duringthe reflection at one of the other mirrors. The two chief ray planesinclude an angle that differs from 0.

According to the invention, it was identified that bothersomepolarization influences can be reduced by virtue of providing a chiefray propagating via at least two chief ray planes which include an anglethat differs from 0. The chief ray of the central object field pointthus no longer runs in precisely one plane. This can be used tocompensate polarization influences on the mirror reflectivity, whichgenerally differ firstly perpendicular and secondly parallel to theplane of incidence on the respective mirror. Defining the respectivechief ray plane by the propagation of the chief ray means that the chiefray of the central object field point incident on the mirror and thechief ray of the central object field point leaving the mirror includean angle that differs from 0 and span the chief ray plane, i.e. bothchief rays lie in the chief ray plane. Bothersome polarizationinfluences, which can be reduced by the optical unit according to theinvention, can emerge as a result of large illumination angles as aresult of large image field-side numerical apertures of the imagingoptical unit. Bothersome polarization influences can emerge during thereflection of imaging light at the mirrors of the optical unit.

The imaging optical unit can have an image-side numerical aperture of atleast 0.4. An image field of the imaging optical unit can have an areawhich is at least 1 mm². An image field of the imaging optical unit canhave an area of more than 1 mm² and can have a lateral dimension that isgreater than 10 mm. Here, the image field is that area on which theimaging optical unit enables imaging with aberrations that are smallerthan prescribed values.

Chief ray planes that are perpendicular to one another were found to beparticularly suitable for reducing bothersome polarization influences.

Precisely two chief ray planes enable a design of the imaging opticalunit which is not too complicated.

An intermediate image in the imaging beam path between the object fieldand the image field makes it possible to influence the angles ofincidence in the beam path profile in the imaging optical unit, whichcan be used as an additional degree of freedom when reducing bothersomepolarization influences. The imaging optical unit can have precisely oneintermediate image. Other embodiments with more than one intermediateimage are also possible.

According to a further aspect, the object specified at the outset isachieved by an imaging catoptric optical unit with at least four mirrorswhich image an object field in an object plane into an image field in animage plane. The imaging optical unit has an image-side numericalaperture of at least 0.4. The imaging optical unit, considered via theimage field, has a maximum diattenuation of 10% for a specific,respectively considered illumination angle.

Here, the diattenuation is defined asD=(u−v)/(u+v),where u denotes the overall reflectivity of all mirrors in the imagingoptical unit for a maximally reflected polarization direction of theimaging light and v denotes the corresponding overall reflectivity forthe polarization of the imaging light perpendicular thereto.

According to the invention, it was identified that the various aspectsof the invention make it possible to realize polarization distributionsof diffraction orders of the illumination that interact during theimaging, which polarization distributions result in either a smalldiattenuation or a diattenuation that prefers a tangential polarizationof the illumination, i.e. in which a tangential polarization componentis reflected at the mirrors of the catoptric optical unit with a greaterreflectivity than a radial polarization component perpendicular thereto.Preferring a tangential polarization reduces bothersome polarizationinfluences during imaging.

According to the invention, it was identified that ray guidance over aplurality of chief ray planes, which include an angle that differs from0, offers an option for reducing bothersome polarization influences. Indoing so, it was identified that it is not mandatory for a diattenuationto be minimized independently of a pupil coordinate or independently ofthe illumination angle. For specific applications it suffices to keep adiattenuation small for respectively a specific absolute illuminationangle, i.e. for all pairs of pupil coordinates with the same radius,i.e. with the same distance from a pupil centre, wherein thediattenuation can by all means differ for various absolute illuminationangles. By way of example, a small maximum diattenuation over all pupilcoordinates can be realized by using an imaging catoptric optical unitwith small maximum angles of incidence on the mirrors of the opticalunit, for example with maximum angles of incidence that are no more than20°, are no more than 15° or are even smaller than that. Particularly inthe region of the maximum image-side numerical aperture, the design ofthe imaging optical unit is, according to the invention, designed suchthat either there is a small maximum diattenuation there, which is lessthan 10%, or that there is a diattenuation there, which prefers apolarization that is tangential to the centre of the pupil of theimaging optical unit. The imaging optical unit can have precisely oneintermediate image. Other embodiments with more than one intermediateimage are also possible. The imaging catoptric optical unit can beembodied as an EUV projection optical unit. An image field of theimaging optical unit can have an area of more than 1 mm² and can have alateral dimension that is greater than 10 mm. Here, the image field isthat area on which the imaging optical unit enables imaging withaberrations that are smaller than prescribed values.

A maximum diattenuation, considered over the image field, of 20% for allpupil coordinates is particularly advantageous.

According to a further aspect, the object specified at the outset isachieved by an imaging optical unit with at least four mirrors whichimage an object field in an object plane into an image field in an imageplane. The imaging optical unit has an image-side numerical aperture ofat least 0.4. The imaging optical unit, considered via the image field,has a diattenuation for a specific illumination angle. The diattenuationattenuating imaging light polarized tangentially to the centre of apupil of the optical unit to a lesser extent than imaging lightpolarized perpendicularly thereto.

The advantages of the imaging optical unit according to precedingparagraph, which prefers a polarization that is tangential to the pupilcentre of the imaging optical unit, which is also referred to astangential diattenuation, correspond to those that were alreadydiscussed above with reference to the imaging optical units according tothe first two aspects. The specific illumination angle, for which atangential diattenuation is present, can be a specific absoluteillumination angle or an illumination angle range about this specificabsolute illumination angle. An annular illumination setting is anexample of such an illumination. The tangential diattenuation can thenbe present for the whole annular illumination setting. A region about aspecific pupil coordinate can also have the tangential diattenuation.There is no need for tangential diattenuation at other illuminationangles. By way of example, in the case of a quadrupole illuminationsetting, individual poles can have a tangential diattenuation whileothers do not. The tangential diattenuation can be present at thelargest illumination angles, i.e. at the edge-side pupil coordinates ofthe imaging optical unit. In the case of small illumination angles inthe region of a centre of the pupil of the imaging optical unit, thediattenuation can deviate from the tangential direction. By way ofexample, the diattenuation in the region of the pupil coordinates thatcover half the numerical aperture from the centre can be at most 20% orat most 10%. A tangential diattenuation can then be present outside thispupil boundary, i.e. towards larger illumination angles. It is notmandatory for the pupil boundary to lie at half the image-side numericalaperture; rather, it can also lie at a different point in the regionbetween 30% and 70% of the numerical aperture.

The features of the imaging optical units according to the invention,explained above, can also be implemented in combination. The specifiedsmall diattenuation values or the diattenuation for preferring atangential polarization can thus be achieved by ray guidance through atleast two chief ray planes which include an angle that differs from 0.

The advantages of an illumination system with an illumination opticalunit for illuminating the object field with illumination or imaginglight and an imaging optical unit described above, a projection exposureapparatus with such an illumination system and a light source forgenerating the illumination or imaging light, a production method ofusing such a projection exposure apparatus, and a micro- ornano-structured component produced by such a method, correspond to thosethat were already explained above with reference to the imaging opticalunit.

Exemplary embodiments of the invention will be explained in more detailbelow on the basis of the drawing. In detail:

FIG. 1 schematically shows a projection exposure apparatus for EUVmicrolithography;

FIG. 2 shows, in a schematic and partly perspective fashion, a catoptricimaging optical unit of the projection exposure apparatus with sixmirrors, wherein a beam path of a chief ray of a central object fieldpoint is shown schematically;

FIG. 3 shows a further embodiment of the catoptric imaging optical unitwith four mirrors;

FIGS. 4a-4b show diagrams and a shading scale in respect of thedependence of a diattenuation on imaging light, which illuminates acentral image field point, depending on the illumination angle at theimaging optical unit according to FIG. 3;

FIG. 4c shows a shading scale;

FIG. 5 shows a further embodiment of the catoptric imaging optical unitwith four mirrors;

FIGS. 6a-6c shows, in an illustration similar to FIG. 4, a dependence ofa diattenuation on imaging light, which illuminates a central imagefield point, depending on the illumination angle at the imaging opticalunit according to FIG. 5;

FIGS. 7 to 9 show, in a perspective view, a mirror arrangement of afurther embodiment of a catoptric imaging optical unit of the projectionexposure apparatus with six mirrors, wherein a beam path of a pluralityof individual rays from, in turn, a plurality of field points is shown;

FIG. 10 shows, in an illustration similar to FIG. 4, a dependence of adiattenuation on imaging light, which illuminates a central image fieldpoint, depending on the illumination angle at the imaging optical unitaccording to FIGS. 7 to 9; and

FIGS. 11 and 12 show two side views of the imaging optical unitaccording to FIGS. 7 to 9.

A projection exposure apparatus 1 for EUV projection lithography has alight source 2 for illumination or imaging light 3. The light source 2is an EUV light source, which produces light in a wavelength range of,for example, between 5 nm and 30 nm, more particularly between 5 nm and10 nm, or around 13.5 nm. A beam path of the illumination light 3 isillustrated very schematically in FIG. 1. An illumination optical unit 6serves to guide the illumination light 3 from the light source 2 to anobject field 4 in an object plane 5. A projection optical unit or animaging optical unit 7 is used to image the object field 4 into an imagefield 8 in an image plane 9 with a predetermined reduction scale. One ofthe exemplary embodiments illustrated in FIG. 2 ff. can be used for theprojection optical unit 7. The projection optical unit 7 according toFIG. 1 has a reduction factor of 4. Other reduction scales are alsopossible, e.g. 4×, 5× or else reduction scales that are greater than 8×.In the projection optical unit 7, the image plane 9 is arranged parallelto the object plane 5. A section of a reflection mask 10, which is alsoreferred to as reticle, that coincides with the object field 4 isimaged. The reflection mask 10 is held by a reticle holder 11.

Imaging by the projection optical unit 7 is brought about on the surfaceof a substrate 12 in the form of a wafer, which is carried by asubstrate holder 13. FIG. 1 schematically illustrates, between thereticle 10 and the projection optical unit 7, a ray beam 14 of theillumination light 3 that enters into the projection optical unit and,between the projection optical unit 7 and the substrate 12, a ray beam15 of the illumination light 3 that emerges from the projection opticalunit 7. An image field-side numerical aperture of the projection opticalunit 7 is 0.4. In FIG. 1, this is reproduced not to scale.

In order to facilitate the description of the projection exposureapparatus 1 and the various embodiments of the projection optical unit7, a Cartesian xyz-coordinate system is specified in the drawings, fromwhich the respective positional relationship of the componentsillustrated in the figures emerges. In FIG. 1, the x-direction runsperpendicular to the plane of the drawing and into the latter. They-direction runs to the right and the z-direction runs downwards.

The projection exposure apparatus 1 is the scanner type. Both thereticle 10 and the wafer 12 are scanned in the y-direction during theoperation of the projection exposure apparatus 1. A stepper-typeprojection exposure apparatus 1, in which there is a stepwisedisplacement of the reticle 10 and the wafer 12 in the y-directionbetween individual exposures of the wafer 12, is also possible.

FIG. 2 schematically shows an embodiment of the projection optical unit7. The beam path of a chief ray 16, of a central object field point,between the object field 4 and the image field 8 is illustrated in FIG.2. The projection optical unit 7 according to FIG. 2 has a total of sixmirrors, which, in the sequence of the beam path of the chief ray 16starting from the object field 4, are numbered M1 to M6 in order.

In FIG. 2, all that is illustrated are schematic sections of thereflection surfaces of the mirrors M1 to M6, with the illustration ofholding structures or support substrates also being dispensed with. Inthe perspective view of FIG. 2, the rear side, facing away from thereflection surface, of the mirror M2 can be seen. The illustrationaccording to FIG. 2 is a meridional section for the mirrors M4 to M6.

The chief ray 16 runs parallel to the yz-plane between the object field4 and the mirror M1. The mirror M1 deflects the chief ray 16 into achief ray plane parallel to the xy-plane. The chief ray 16 runs parallelto the xy-plane between the mirrors M1 and M4. The mirror M4 deflectsthe chief ray 16 from the chief ray plane parallel to the xy-plane tothe chief ray plane parallel to the yz-plane. The chief ray 16 runsparallel to the yz-plane between the mirror M4 and the image field 8,with the yz-profile plane of the chief ray 16 between mirror M4 and theimage field 8 coinciding with the yz-profile plane between the objectfield 4 and the mirror M1.

The mirror M6 is obscured, i.e. it has a passage opening 17 for theimaging light 3 in the beam path between the mirrors M4 and M5.

A first chief ray plane of the imaging optical unit 7 according to FIG.2 is prescribed by the profile of the chief ray 16 during the reflectionat the mirror M5. The chief ray section 16 _(M5) incident on the mirrorM5 and the chief ray section 16 _(M6) leaving the mirror M5 include anangle α that differs from 0 and therefore span the first yz-chief rayplane.

A second chief ray plane is prescribed by the profile of the chief ray16 during the reflection at the mirror M2. The two chief ray sections 16_(M2) and 16 _(M3) reflected there likewise include an angle thatdiffers from 0 and span the second yz-chief ray plane parallel to thexy-plane.

The two chief ray planes, which are prescribed by the mirrors M5 and M2and are parallel to the yz-plane and parallel to the xy-plane, includean angle that differs from 0, specifically they are perpendicular to oneanother.

The imaging optical unit 7 according to FIG. 2 has precisely two chiefray planes.

As a result of the imaging light 3 running through two chief ray planeswhich include an angle that differs from 0, an equalization of adiattenuation of the imaging light 3 is achieved when passing throughthe imaging optical unit 7.

The imaging light 3 has polarization components firstly in the xy-planeand secondly in the yz-plane. The valueD=(u−v)/(u+v)is referred to as diattenuation of the imaging optical unit 7, where udenotes the overall reflectivity of all mirrors M1 to M6 in the imagingoptical unit for the maximally reflected polarization direction and vdenotes the corresponding overall reflectivity for the polarizationperpendicular thereto.

For a respectively considered absolute illumination angle, with whichany image field point of the image field 8 of the imaging optical unit 7is illuminated, the imaging optical unit 7 according to FIG. 2 has amaximum diattenuation of 10%.

The illumination angle is measured starting from a normal, penetratingthe central image field point, on the image plane 9.

The imaging optical unit 7 according to FIG. 2 can be configured suchthat, considered over the whole image field 8, it has a maximumdiattenuation of 20% for all illumination angles.

FIG. 3 shows a further embodiment of the imaging optical unit 7. Thisembodiment has not been optimized for projection purposes and serves forexplaining the principle. In addition to the profile of the chief ray 16of the central object field point, the profile of a few further imagingrays 18 have also been illustrated, the latter belonging to variouspupil coordinates or illumination angles of the central image fieldpoint.

The imaging optical unit 7 according to FIG. 3 has a total of fourmirrors which, in the sequence of the beam path of the individual rays16, 18 starting from the object field 4, are numbered M1 to M4 in order.Once again, it is sections of the reflection surfaces without holdingstructures and substrates that are illustrated. The mirrors M1 to M4carry a bi-layer coating in the form of a molybdenum/silicon bi-layer.The imaging optical unit 7 according to FIG. 3 is designed for a usedwavelength of 13.5 nm. An imaging scale of the imaging optical unit 7according to FIG. 3 is 1×. An image field-side aperture is 0.2.

Between the object field 4 and the mirror M3, the chief ray 16 runs in afirst chief ray plane, which runs parallel to the yz-plane. This firstyz-chief ray plane is prescribed by the profile of the chief ray 16during the reflection at, for example, the mirrors M1 and M2, as alreadyexplained above in the context of the embodiment according to FIG. 2.

The mirror M3 deflects the chief ray 16 out of the first chief ray planeyz, with the chief ray 16, following the reflection at the mirror M3,running in the xz-plane up to the image field 8. The mirror M4 isarranged outside the yz-plane and can be situated in front of or behindthe plane of the drawing of FIG. 3.

The chief ray 16 runs parallel to the z-axis between the mirror M4 andthe image field 8.

The second chief ray plane of the imaging optical unit 7 according toFIG. 3, i.e. the plane parallel to the xz-axis, is prescribed by theprofile of the chief ray 16 during the reflection at, for example, themirrors M3 and M4.

FIG. 4 shows the dependence of the diattenuation D on the illuminationangle of the central image field point.

In FIG. 4b , the diattenuation D (b_(x), b_(y)) is plotted at therespective pupil coordinates b_(x), b_(y), for example for an entrancepupil of the optical unit 7 according to FIG. 3. Pupil coordinates withthe same radius, i.e. the same distance from the origin of thecoordinate system according to FIGS. 4a and 4b , which coincides with acentre of the entrance pupil of the optical unit 7, belong to the sameillumination angle. The value b_(x) ² b_(y) ² thus constitutes a measurefor an absolute illumination angle, measured starting from a normal onthe image plane through the central image field point, wherein theangles of the respectively considered illumination angle with respect tothis normal, firstly measured in the xz-plane and secondly measured inthe yz-plane, are the values b_(x), b_(y) of the pupil coordinates inunits of the image field-side numerical aperture of the imaging opticalunit 7 according to FIG. 3. The diattenuation D is plotted inpercentages in FIGS. 4a and 4 b.

The respective value D dependent on the pupil coordinates b_(x), b_(y)is indicated by a shading scale, which is specified in FIG. 4c . In thecentre of FIGS. 4a and 4b , i.e. at the smallest pupil coordinates, inthe region of a perpendicular illumination, the diattenuation is small.Towards the outside, i.e. to larger absolute illumination angles, thediattenuation D (b_(x), b_(y)) increases, with the value profile of thediattenuation being approximately rotationally symmetric. In the case ofa given absolute numerical aperture, i.e. in the case of a respectivelyconsidered absolute illumination angle, a variation of the diattenuationD about a mean diattenuation value at this absolute illumination angleis therefore small and is, apart from illumination angles in the regionof the maximum image field-side numerical aperture, less than 20% andless than 10% for even smaller illumination angles. In the illustrationaccording to FIG. 4b , the diattenuation at a respectively consideredillumination angle varies by less than 10% from a mean diattenuationvalue at this illumination angle.

In addition to the diattenuation D plotted in terms of absolute value,FIG. 4a at the respective pupil coordinates b_(x), b_(y) also indicatesthe profile of that polarization direction which is preferably reflectedby the mirrors M1 to M4 of the imaging optical unit 7. In the region ofthe second quadrant in FIG. 4a , i.e. at negative b_(x)- and positiveb_(y)-values, there is, to a good approximation, a preference for thetangential polarization about the pupil centre. This approximation ofpreference for the tangential polarization, with restrictions, alsostill applies to the first and the third pupil coordinate quadrants inFIG. 4a . The preference for the tangential polarization is advantageousfor imaging even if the absolute value for the diattenuation is greaterthan 20% for example. Boundary lines between the transitions of thediattenuation values as per the shading scale in FIG. 4c are once againclarified by solid lines in FIG. 4 a.

FIGS. 5 and 6 are used below to explain a further embodiment of theimaging optical unit 7. Components and functions that correspond tothose that were already explained above with reference to FIGS. 1 to 4,and in particular with reference to FIGS. 3 and 4, have the samereference signs and will not be discussed again in detail.

In contrast to the imaging optical unit 7 according to FIG. 3, theimaging optical unit 7 according to FIG. 5 has an intermediate image 19in the beam path between the mirrors M2 and M3. As a result of this,there is a change in the angle of incidence distribution of the imagingrays 18, particularly on the subsequent mirrors M3 and M4. This leads toa correspondingly modified influence, particularly of mirrors M3 and M4,on the reflectivities of the polarization components of the imaginglight 3, firstly parallel to the xz-plane and secondly parallel to theyz-plane.

FIGS. 6a-6c in turn show the resultant diattenuation for the wholeimaging optical unit 7 according to FIG. 5. Apart from the largestillumination angles, there is an overall very small diattenuation, whichis practically vanishing for a majority of the illumination angles. Adiattenuation greater than 15% is present only in the region of thefirst quadrant of the pupil coordinates according to FIG. 6b , i.e. inthe region of positive b_(x)- and positive b_(y)-values. A diattenuationof no more than 20% is present at practically all pupil coordinates.

Here, the described embodiments of the imaging optical unit 7 arecatoptric optical units in each case, i.e. pure mirror optical unitswithout refractive components.

FIGS. 7 to 12 are used to explain a further embodiment of the imagingoptical unit 7 below. Components and functions that correspond to thosethat were already explained above with reference to FIGS. 1 to 6 and inparticular with reference to FIGS. 3 and 4 bear the same reference signsand will not be discussed again in detail. The Cartesian xyz-coordinatesystem used below to describe positional relationships of the componentsof the imaging optical unit 7 according to FIG. 7 ff. is rotated aboutthe z-axis by 90° compared to the xyz-coordinate system used above withreference to FIGS. 1 to 6, and so scanning of the reticle 10 and of thewafer 12 now takes place in the x-direction.

The imaging optical unit 7 according to FIGS. 7 to 12 has a total of sixmirrors, which are numbered M1 to M6 in order in the sequence in whichthey are struck by the illumination light 3 in the imaging beam pathbetween the object field 4 and the image field 8. Shown here is theimaging beam path of a plurality of individual rays of the illuminationlight 3, which in turn start from a plurality of object field points.The image field 8 has field dimensions of 2 mm in the x-direction and of26 mm in the y-direction. The object field 4 accordingly has dimensionsenlarged by a factor 4 in both the x- and in the y-direction. Theimaging optical unit 7 according to FIG. 7 ff. thus provides a reductionby a factor 4 between the object field 4 and the image field 8. Attachedto the mirror M2 is an aperture stop for restricting the beam of theillumination or imaging light 3. This aperture stop can be embodied as acoating on the mirror M2.

The mirrors M1 to M4 lie in a common plane, which runs perpendicular tothe xz-plane and is tilted to the yz-plane. The mirrors M3 to M6 and theimage field 8 are arranged in a second plane, which runs parallel to thexz-plane. The object field 4 and the mirrors M1 and M2 also lie in aplane which is parallel to the xz-plane and spaced apart from the planein which the mirrors M3 to M6 lie. The chief ray plane yz and the chiefray plane xz are part of a Cartesian xyz-coordinate system and includean angle of 90°, i.e. they are perpendicular to one another. There is anintermediate image 19 in the imaging beam path between the mirrors M4and M5. Spatially, the intermediate image is situated in the region of apassage opening 20 in the last mirror M6, through which passage openingthe illumination light 3, which is routed between the mirrors M4 and M5,passes through the mirror M6.

The imaging optical unit 7 according to FIGS. 7 to 12 has an image-sidenumerical aperture of 0.45. A reduction scale of the imaging opticalunit 7 according to FIGS. 7 to 12 is 4×. A chief ray angle CRA (compareFIG. 12) of the imaging light 3 to the normal to the object plane 5 is9.5° for a central field point of the object field 4.

The optical design of the imaging optical unit 7 according to FIGS. 7 to12 is described below on the basis of design data from the opticaldesign program CODE V®.

The freeform reflection surfaces of the mirrors M1 to M6 are describedby the following equation:

$Z = {\frac{{cr}^{2}}{1 + \sqrt{1 - {\left( {1 + k} \right)c^{2}r^{2}}}} + {\sum\limits_{i,j}{c_{ij}x^{i}y^{j}}}}$Z is the arrow height of the freeform surface at the point x, y(x²+y²=r²).c is a constant corresponding to the vertex curvature of a correspondingaspheric lens. k corresponds to a conical constant of a correspondingaspheric lens. c_(ij) are the coefficients of the monomials x^(i)y^(i).The values of c, k and c_(ij) are typically determined on the basis ofthe desired optical properties of the mirror within the imaging opticalunit 7.

Freeform surfaces can also be described mathematically by Zernikepolynomials, which, for example, are explained in the manual of theoptical design program CODE V®. Alternatively, freeform surfaces canalso be described with the aid of two-dimensional spline surfaces.Examples of this are Bezier curves or non-uniform rational basis splines(NURBS). By way of example, two-dimensional spline surfaces can bedescribed by a grid of points in an xy-plane and associated z-values, orby these points and gradients associated therewith. Depending on therespective type of the spline surface, the complete surface is obtainedby interpolation between the grid points using e.g. polynomials orfunctions that have specific properties in respect of their continuityand differentiability. Examples of this are analytic functions.

The mirrors M1 to M6 carry multiple reflection layers for optimizingtheir reflection for the incident EUV illumination light 3. Theoptimization of the reflection can be improved the closer the impactangles of the individual rays of the illumination or imaging light 3 onthe mirror surfaces are to perpendicular incidence.

The first of the following tables (Table 1) of the optical designrespectively specifies the reciprocal of a vertex curvature (radius) forthe optical surfaces, i.e. for the reflection surfaces of the mirrors M1to M6.

The second of the following tables (Table 2) specifies decentring andinclination or tilt values of the mirrors M1 to M6 in the form oftranslation parameters XDE, YDE, ZDE and rotation parameters ADE, BDE,CDE.

The meaning of these parameters corresponds to those which are knownfrom the optical design program CODE V®. This meaning will once again beexplained briefly below. It should be noted that in respect ofdecentring, an additional rotation of 180° about the y-axis is stillundertaken in contrast to the descriptions known from CODE V®. Thisleads to positive distance values between the mirrors or between thereference surfaces. When defining the ray intersection side using CODEV®, the ray intersection side (SID) is to be set to “NEG”. Such rayintersection side (SID) parameter is described e.g. on page 4-60 ff inthe CODE V® 10.4 reference manual, Volume I, September 2011.

-   ADE Rotation of the surface by angle alpha in degrees about the    x-axis.-   BDE Rotation of the surface by angle beta in degrees about the    y′-axis which then emerges from rotating the y-axis.-   CDE Rotation of the surface by angle gamma in degrees about the    z″-axis which has emerged from the z-axis by firstly rotation about    the x-axis and secondly about the y′-axis.-   XDE Translation of the surface in the x-axis in mm.-   YDE Translation of the surface in the y-axis in mm.-   ZDE Translation of the surface in the z-axis in mm.

The third following table (Tables 3a and 3b) specifies the coefficientsc_(ij) of the monomials x^(i)y^(i) in the aforementioned freeformsurface equation for mirrors M1 to M6.

TABLE 1 Surface Radius Object plane infinity M1 −2648.044184 M222960.104483 M3 2860.372482 M4 −1815.779736 M5 504.945215 M6 −706.456328Image plane infinity

TABLE 2 Decentring and tilt angle Surface XDE YDE ZDE ADE BDE CDE Objectplane 0.000000 0.000000 0.000000 0.000000 0.000000 0.000000 M1305.573789 −34.039631 1817.972774 4.193642 −5.815188 −8.157803 M2−472.877892 −33.820402 448.867511 −10.210273 −191.664381 12.284104 M3145.260385 −481.631793 1464.173836 −12.162098 −30.754773 −4.726896 M4−481.815240 −481.508915 431.654420 −0.002389 −203.368069 −22.476721 M5−18.782074 −481.355118 2103.784831 0.007160 −15.226020 −3.316004 M6−159.869266 −481.417847 1578.096249 −0.207089 −187.104105 −55.421103Image plane 0.000000 0.000000 2192.524695 0.000000 0.000000 0.000000

TABLE 3a Coefficient M1 M2 M3 X −2.552485E−01 −3.283324E−01 3.131364E−04Y 4.513489E−02 4.336789E−02 −1.886828E−03 X2 −2.496545E−05 −5.748374E−069.804823E−05 XY −4.709745E−06 2.929715E−05 9.785084E−05 Y2 1.275106E−05−1.623710E−06 −1.753080E−04 X3 −7.490557E−09 2.141492E−07 5.344556E−07X2Y 1.360743E−08 −3.228143E−07 3.290905E−07 XY2 7.759237E−097.055326E−08 1.255602E−07 Y3 5.385756E−09 −1.030819E−08 2.273968E−08 X4−7.423991E−12 2.740267E−10 1.161193E−09 X3Y −7.789197E−12 −5.149048E−108.071598E−10 X2Y2 −1.803679E−12 5.208166E−10 3.386959E−10 XY33.123165E−12 −3.672083E−12 1.698773E−10 Y4 1.585952E−12 6.322235E−114.792957E−12 X5 −1.122002E−14 1.360011E−13 1.346860E−12 X4Y−2.561007E−15 −9.472242E−13 1.692105E−12 X3Y2 −7.341755E−15 7.270213E−131.144118E−12 X2Y3 2.608413E−15 −5.418624E−13 4.650047E−13 XY4−4.546959E−15 8.326234E−14 −7.633684E−14 Y5 2.209581E−15 4.898049E−14−2.660709E−17 X6 −2.088419E−17 −7.574627E−17 5.063545E−15 X5Y4.067591E−17 −2.752801E−16 −6.847713E−15 X4Y2 −3.209935E−17 2.128181E−15−3.484772E−15 X3Y3 1.326889E−17 2.496004E−18 −1.727155E−15 X2Y42.005796E−17 1.642674E−15 −7.402284E−16 XY5 −2.153712E−17 8.619971E−16−7.023807E−17 Y6 7.031581E−18 3.018558E−16 −5.904251E−17 X7−7.095014E−21 −2.184083E−18 3.534016E−17 X6Y −1.877551E−20 8.069367E−18−6.458748E−17 X5Y2 −1.782909E−21 −7.171148E−18 −8.961763E−17 X4Y3−7.272771E−20 7.334482E−18 −3.758514E−17 X3Y4 −3.683629E−20−3.229648E−18 −7.602384E−18 X2Y5 −4.080520E−20 1.732968E−18−5.070519E−19 XY6 3.555920E−21 −6.530600E−19 1.541444E−19 Y7−1.581699E−22 4.344477E−19 −1.439807E−20 X8 5.154333E−23 −8.262628E−213.810649E−19 X7Y −2.121857E−22 3.205304E−20 8.619901E−19 X6Y22.711810E−22 −7.302157E−20 9.377840E−20 X5Y3 −2.631038E−22 2.993325E−20−9.406916E−20 X4Y4 2.505609E−24 −5.959552E−20 −1.319137E−20 X3Y5−1.414276E−22 2.265139E−21 3.000266E−20 X2Y6 −9.459223E−23 −2.578485E−207.130840E−21 XY7 9.302010E−23 −2.338337E−22 9.073679E−22 Y8 5.917445E−24−4.292258E−21 4.918382E−22 X9 −1.176898E−25 −2.538973E−23 7.258883E−22X8Y 2.560439E−26 1.329617E−23 2.680452E−21 X7Y2 −8.949191E−25−1.551430E−22 7.383072E−23 X6Y3 5.934517E−25 7.621236E−24 −8.377442E−22X5Y4 −4.991909E−25 −4.131256E−23 −3.204037E−22 X4Y5 1.197862E−242.256452E−23 −1.728816E−22 X3Y6 −5.532085E−25 −4.334289E−24 1.235500E−23X2Y7 3.720874E−25 1.094273E−23 −1.235466E−23 XY8 −1.022775E−251.163846E−23 1.928222E−25 Y9 −2.879262E−26 −6.379814E−26 7.015097E−25X10 −2.720299E−28 3.924841E−26 −1.481697E−23 X9Y −6.388986E−28−3.155426E−25 −1.717157E−24 X8Y2 −1.336772E−27 4.490411E−25 8.863983E−24X7Y3 5.266971E−28 1.365593E−25 7.397201E−24 X6Y4 −1.106860E−271.097812E−24 3.260299E−24 X5Y5 1.343277E−27 3.451172E−25 3.640949E−24X4Y6 4.127581E−28 6.954782E−25 7.862214E−25 X3Y7 4.593674E−281.935475E−25 1.042095E−25 X2Y8 −5.285495E−28 3.436546E−25 3.413288E−26XY9 −4.488550E−29 1.481782E−26 1.378870E−26 Y10 −3.612161E−285.046107E−26 1.875209E−27

TABLE 3b Coefficient M4 M5 M6 X 3.1313295e−04 −3.2833235e−01−2.5524851e−01 Y −1.8868263e−03 4.3367887e−02 4.5134887e−02 X29.8048309e−05 −5.7483741e−06 −2.4965449e−05 XY 9.7850455e−052.9297145e−05 −4.7097450e−06 Y2 −1.7530816e−04 −1.6237101e−061.2751059e−05 X3 5.3446084e−07 2.1414921e−07 −7.4905569e−09 X2Y3.2908732e−07 −3.2281434e−07 1.3607432e−08 XY2 1.2555383e−077.0553263e−08 7.7592372e−09 Y3 2.2737643e−08 −1.0308189e−085.3857560e−09 X4 1.1613874e−09 2.7402672e−10 −7.4239910e−12 X3Y8.0700934e−10 −5.1490483e−10 −7.7891966e−12 X2Y2 3.3865521e−105.2081656e−10 −1.8036792e−12 XY3 1.6982361e−10 −3.6720828e−123.1231649e−12 Y4 4.7837012e−12 6.3222351e−11 1.5859521e−12 X51.3471858e−12 1.3600111e−13 −1.1220017e−14 X4Y 1.6922399e−12−9.4722422e−13 −2.5610074e−15 X3Y2 1.1437278e−12 7.2702125e−13−7.3417550e−15 X2Y3 4.6446691e−13 −5.4186243e−13 2.6084134e−15 XY4−7.6421462e−14 8.3262341e−14 −4.5469587e−15 Y5 −6.3318807e−174.8980492e−14 2.2095811e−15 X6 5.0691128e−15 −7.5746266e−17−2.0884187e−17 X5Y −6.8487678e−15 −2.7528009e−16 4.0675908e−17 X4Y2−3.4859387e−15 2.1281811e−15 −3.2099354e−17 X3Y3 −1.7279599e−152.4960037e−18 1.3268892e−17 X2Y4 −7.4219257e−16 1.6426744e−152.0057957e−17 XY5 −7.1024385e−17 8.6199705e−16 −2.1537122e−17 Y6−5.9080464e−17 3.0185584e−16 7.0315808e−18 X7 3.5338591e−17−2.1840832e−18 −7.0950136e−21 X6Y −6.4585653e−17 8.0693674e−18−1.8775513e−20 X5Y2 −8.9619875e−17 −7.1711477e−18 −1.7829094e−21 X4Y3−3.7588002e−17 7.3344820e−18 −7.2727713e−20 X3Y4 −7.6068348e−18−3.2296479e−18 −3.6836285e−20 X2Y5 −5.1827598e−19 1.7329676e−18−4.0805202e−20 XY6 1.4725458e−19 −6.5306000e−19 3.5559196e−21 Y7−1.4432459e−20 4.3444774e−19 −1.5816986e−22 X8 3.8107526e−19−8.2626277e−21 5.1543330e−23 X7Y 8.6198715e−19 3.2053045e−20−2.1218570e−22 X6Y2 9.3778937e−20 −7.3021573e−20 2.7118099e−22 X5Y3−9.4069713e−20 2.9933251e−20 −2.6310380e−22 X4Y4 −1.3213626e−20−5.9595523e−20 2.5056093e−24 X3Y5 2.9980985e−20 2.2651387e−21−1.4142760e−22 X2Y6 7.0871395e−21 −2.5784850e−20 −9.4592234e−23 XY78.8225536e−22 −2.3383368e−22 9.3020100e−23 Y8 4.9269306e−22−4.2922579e−21 5.9174451e−24 X9 7.2587806e−22 −2.5389727e−23−1.1768981e−25 X8Y 2.6804560e−21 1.3296168e−23 2.5604395e−26 X7Y27.3828243e−23 −1.5514297e−22 −8.9491910e−25 X6Y3 −8.3774096e−227.6212361e−24 5.9345172e−25 X5Y4 −3.2041644e−22 −4.1312559e−23−4.9919093e−25 X4Y5 −1.7289215e−22 2.2564517e−23 1.1978624e−24 X3Y61.2239484e−23 −4.3342887e−24 −5.5320845e−25 X2Y7 −1.2424610e−231.0942726e−23 3.7208741e−25 XY8 8.1918056e−26 1.1638455e−23−1.0227747e−25 Y9 7.2627769e−25 −6.3798141e−26 −2.8792617e−26 X10−1.4816945e−23 3.9248408e−26 −2.7202993e−28 X9Y −1.7171642e−24−3.1554260e−25 −6.3889862e−28 X8Y2 8.8639860e−24 4.4904114e−25−1.3367716e−27 X7Y3 7.3972005e−24 1.3655930e−25 5.2669713e−28 X6Y43.2602910e−24 1.0978122e−24 −1.1068600e−27 X5Y5 3.6409604e−243.4511721e−25 1.3432774e−27 X4Y6 7.8607032e−25 6.9547820e−254.1275809e−28 X3Y7 1.0362932e−25 1.9354752e−25 4.5936739e−28 X2Y83.3799267e−26 3.4365460e−25 −5.2854950e−28 XY9 1.3347505e−261.4817817e−26 −4.4885497e−29 Y10 1.8094176e−27 5.0461065e−26−3.6121608e−28

The mirrors M1 to M5 each have no passage opening for the illuminationlight 3.

In sections, the mirrors M3 and M6 are situated back-to-back.

FIG. 10 shows the resultant diattenuation for the whole imaging opticalunit 7 according to FIGS. 7 to 12. Apart from the largest illuminationangles, there is, overall, a very small diattenuation, which ispractically vanishing for a majority of the illumination angles. Thediattenuation is in each case less than 20% in the region of theobserved pupil coordinates.

In order to produce a micro- or nano-structured component, in particulara semiconductor component in the form of a microchip, in particular amemory chip, the projection exposure apparatus 1 is used as follows:initially the reflection mask 10 and the substrate 12 are provided. Astructure on the reticle 10 is subsequently projected onto alight-sensitive layer of the wafer 12 with the aid of the projectionexposure apparatus 1. A micro- or nano-structure is then produced on thewafer 12 and hence the micro-structured component is produced bydeveloping the light-sensitive layer.

The invention claimed is:
 1. An imaging optical unit, comprising: aplurality of mirrors configured so that, during use of the imagingoptical unit, the plurality of mirrors images an object field in anobject plane into an image field in an image plane, wherein: theplurality of mirrors comprises first, second, third and fourth mirrors;a first chief ray plane is defined by propagation of a chief ray of acentral object field point during reflection at the first mirror; asecond chief ray plane is defined by propagation of the chief ray of thecentral object field point during the reflection at the second mirror;the first and second chief ray planes define an angle that differs from0; and the imaging optical unit is an imaging catoptric EUV projectionoptical unit.
 2. The imaging optical unit of claim 1, wherein the firstchief ray plane is perpendicular to the second chief ray plane.
 3. Theimaging optical unit of claim 2, wherein the imaging optical unit hasprecisely two chief ray planes.
 4. The imaging optical unit of claim 3,wherein the imaging optical unit has an intermediate image in an imagingbeam path between the object field and the image field.
 5. The imagingoptical unit of claim 1, wherein the imaging optical unit has preciselytwo chief ray planes.
 6. The imaging optical unit of claim 5, whereinthe imaging optical unit has an intermediate image in an imaging beampath between the object field and the image field.
 7. The imagingoptical unit of claim 1, wherein the imaging optical unit has anintermediate image in an imaging beam path between the object field andthe image field.
 8. The imaging optical unit of claim 7, wherein thefirst chief ray plane is perpendicular to the second chief ray plane. 9.An illumination system, comprising: an illumination optical unit; and animaging optical unit according to claim 1, wherein the illuminationoptical unit is configured to illuminate the object field.
 10. Anapparatus, comprising: a light source; and an illumination system whichcomprises: an illumination optical unit; and an imaging optical unitaccording to claim 1, wherein the illumination optical unit isconfigured to illuminate the object field with light generated by thelight source, and the apparatus is a projection exposure apparatus. 11.A method of using a microlithographic projection exposure apparatuscomprising an illumination optical unit and an imaging optical unit, themethod comprising: using the illumination optical unit to illuminate areticle comprising structures; and using the imaging optical unit toproject a portion of the reticle onto a light-sensitive material,wherein the imaging optical unit comprises an imaging optical unitaccording to claim
 1. 12. The imaging optical unit of claim 1, wherein:the imaging optical unit has an image-side numerical aperture of atleast 0.4; considered via the image field, the imaging optical unit hasa diattenuation for a specific illumination angle; and the diattenuationattenuating imaging light polarized tangentially to the center of apupil of the optical imaging unit to a lesser extent than imaging lightpolarized perpendicularly thereto.
 13. An illumination system,comprising: an illumination optical unit; and an imaging optical unitaccording to claim 12, wherein the illumination optical unit isconfigured to illuminate the object field.
 14. An apparatus, comprising:a light source; and an illumination system which comprises: anillumination optical unit; and an imaging optical unit according toclaim 12, wherein the illumination optical unit is configured toilluminate the object field with light generated by the light source,and the apparatus is a projection exposure apparatus.
 15. A method ofusing a microlithographic projection exposure apparatus comprising anillumination optical unit and an imaging optical unit, the methodcomprising: using the illumination optical unit to illuminate a reticlecomprising structures; and using the imaging optical unit to project aportion of the reticle onto a light-sensitive material, wherein theimaging optical unit comprises an imaging optical unit according toclaim
 12. 16. An imaging optical unit, comprising: at least four mirrorsconfigured so that, during use of the imaging optical unit, the at leastfour mirrors image an object field in an object plane into an imagefield in an image plane, wherein: the imaging optical unit has animage-side numerical aperture of at least 0.4; considered via the imagefield, the imaging optical unit has a maximum diattenuation of 10% for aspecific, respectively considered illumination angle; and the imagingoptical unit is an imaging catoptric optical unit.
 17. The imagingoptical unit of claim 16, wherein, considered over the image field, theimaging optical unit has a maximum diattenuation of 20% for all pupilcoordinates.
 18. An illumination system, comprising: an illuminationoptical unit; and an imaging optical unit according to claim 16, whereinthe illumination optical unit is configured to illuminate the objectfield.
 19. An apparatus, comprising: a light source; and an illuminationsystem which comprises: an illumination optical unit; and an imagingoptical unit according to claim 16, wherein the illumination opticalunit is configured to illuminate the object field with light generatedby the light source, and the apparatus is a projection exposureapparatus.
 20. A method of using a microlithographic projection exposureapparatus comprising an illumination optical unit and an imaging opticalunit, the method comprising: using the illumination optical unit toilluminate a reticle comprising structures; and using the imagingoptical unit to project a portion of the reticle onto a light-sensitivematerial, wherein the imaging optical unit comprises an imaging opticalunit according to claim 16.